The present disclosure relates to process variable transmitters used in process control and monitoring systems. More specifically, the present disclosure relates to using a component embedded in a transmitter to detect sensor degradation or failure.
Process variable transmitters are used to measure process parameters in a process control monitoring system. Microprocessor-based transmitters often include a sensor, an analog-to-digital converter for converting an output from the sensor into a digital form, a microprocessor for compensating the digitized output, and an output circuit for transmitting the compensated output. Currently, this transmission is normally done over a process control loop, such as a 4-20 milliamp control loop, or wirelessly.
Such process variable transmitters are often connected to a host system by the process control loop. Some current host systems use a statistic, referred to as the Standard Error of the Estimate (Se) statistic, in order to analyze measurement data to predict whether sensors or other process components are failing, before such failures become critical. For instance, some such host systems use the Se statistic to determine whether one of the sensors (such as a thermocouple) is degraded but not yet failing. Other host systems use the Se statistic in order to identify certain process failures (such as reactor runaway) before severe damage occurs. In order to provide this type of information with sufficient accuracy, the conventional Se equation must be updated at least once every second, or more frequently.
More specifically, the Se statistic is used to determine the standard deviation of a sensor signal over a linearization region. This helps to identify how well the data is linearized or how well it is tracking the linearization. One example of the equations used for this statistic are shown below:
                    A        =                              12            *                          ∑                              i                ⁡                                  (                                                            T                      i                                        -                    T                                    )                                                                                        n              ⁡                              (                                                      n                    2                                    -                  1                                )                                      ⁢            Δ            ⁢                                                  ⁢            t                                              Eq        .                                  ⁢        1                                B        =                              T            _                    -                                                    A                ⁡                                  (                                      n                    +                    1                                    )                                            ⁢              Δ              ⁢                                                          ⁢              t                        2                                              Eq        .                                  ⁢        2                                                      T            .                    i                =                              Ai            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            t                    +          B                                    Eq        .                                  ⁢        3                                          S          6                =                                            ∑                                                (                                                            T                      i                                        -                                                                  T                        .                                            i                                                        )                                2                                                    (                              n                -                2                            )                                                          Eq        .                                  ⁢        4            
Where:
n=number of samples (usually 12 to 20)
i=sample number (1 to n)
 Ti=n average inputs
{dot over (T)}i=calculated input
Ti=sampled_inputs
t=update_time
Se=Standard Error of Estimate
These equations are resource intensive for embedded microcontroller systems used on many process transmitters. They require a relatively large amount of random access memory for each sensor that is being analyzed. The resolution of the equations is also quite time intensive, and this can reduce the amount of time that is available to process the normal measurement signal input by the sensor.